Fabrication of ultra-large scale integrated circuits involves deposition of metallic films using physical vapor deposition (PVD). Typically, a target is provided consisting of the material which is to be deposited as the thin film onto a workpiece or semiconductor wafer. The material may be copper, titanium, tantalum or other metal, metal oxide, metal nitride, for example. In one process, for example, titanium nitride is deposited onto a thin film structure that includes a very thin gate oxide layer of HfO2 overlying a source-drain channel. Such processes are required to achieve a highly uniform distribution of deposited film thickness across the entire workpiece or wafer. Currently, PVD processes rely on short target wafer spacing, typical less than 100 mm or wafer backside bias to achieve good uniformity. However, many processes, especially processes for front-end application, require that no plasma damage be induced during the deposition. Both short target wafer spacing and wafer bias will create plasma damage on the wafer. With target wafer spacing longer than 110 mm and zero watt wafer bias, PVD processes are capable of achieving a deposited film thickness uniformity across a 300 mm diameter wafer in which the standard deviation in film thickness is about 6%. As feature size or critical dimension is being reduced down to 32 nm and below, the film thickness uniformity requirement is becoming more stringent, with the allowable standard deviation in film thickness being reduced to 1%. Current PVD processes are not capable of attaining such a high degree of uniformity on a reliable basis.
A conventional PVD reactor includes a vacuum chamber, a sputter target (copper, titanium, tantalum or other desired metal) at the reactor chamber ceiling, a support pedestal for holding the workpiece (e.g., semiconductor wafer) below and facing the ceiling, a high voltage D.C. power supply coupled to the target and a gas injection apparatus for introducing a carrier gas (e.g., argon) into the reactor chamber. The D.C. voltage on the target is sufficient to ionize the carrier gas to produce a plasma near the sputter target. A magnetron assembly consisting of a rotating magnet overlies the ceiling and the sputter target, and creates a sufficiently high magnetic field to confine the plasma near the target to produce plasma sputtering of the target. The material sputtered from the target may include both neutrals and ions of the target species, and a portion of the sputtered material deposits onto the workpiece as a thin film. In some cases, D.C. or RF bias power may be coupled to the workpiece to attract ions sputtered from the target.
The target erodes in an area covered by the magnetron. During deposition, the magnetron is moved across the ceiling in a circular or planetary motion, to distribute the target erosion and to distribute the deposition across the workpiece. However, the deposition rate distribution across the workpiece tends to be high at the center of the workpiece and low at the edge, limiting the uniformity so that the minimum deviation in deposited film thickness is in excess of 5%.